1. Field of the Invention
This invention relates to radar systems (and sonar and ladar) and methods for determining the range of objects, and more particularly to radar systems and methods for accurately determining the range of objects having little or no relative velocity.
2. Description of Related Art
Radio Detection and Ranging ("Radar") is commonly employed to detect and determine the range of objects or targets relative to the radar system. FIG. 1 is a diagram of a general radar system 1 and a channel or medium 2 that includes a target 30. As shown in FIG. 1, the radar system includes a transmitter 10 having a transmit antenna 12 and a receiver 20 having a receive antenna 22. In simple terms, the transmitter 10 generates a signal s(t) that is converted to an electromagnetic wave 14 by the transmit antenna 12. The signal travels at the speed of light, c away from the transmit antenna 12 in the medium of the channel 2. The signal may reflect off targets or objects such as the target 30 in the channel 2. The receive antenna 22 receives the reflected electromagnetic waves and generates a signal s.sub.r (t), which is processed by the receiver 20. It is noted that the transmit antenna 12 and the receive antenna 22 may be in close proximity (monostatic radar systems). Alternatively, the transmitter 10 and the receiver 20 may be separated by a large distance (e.g., in bistatic radar systems).
In radar systems, if s(t) is a pulsed signal, the received signal s.sub.r (t) is nominally equal to .alpha.s(t-t.sub.r). In such systems, t.sub.r is the round trip delay or the time required for the electromagnetic wave to travel from the radar transmit antenna to the target and back to the receive antenna and ox is an amplitude scaling coefficient. In such systems the range of the target is nominally equal to c.times.t.sub.r /2 where c is the speed of light (approximately equal to 3(10.sup.8)m/s in a vacuum). If the target is moving away from or toward the radar system (ie., has a non-zero relative velocity), the relative velocity of the target may be determined by calculating the frequency or Doppler shift of s(t). In particular, it is well known that the velocity of the target, v, is nominally equal to -f.sub.d *c/f.sub.0 where f.sub.d is the Doppler frequency and f.sub.0 is the frequency of the transmitted wave 14 of s(t). These principles also apply to sonar and ladar (laser-based) target detection and ranging systems. In radar the velocity of propagation is also the speed of light (the same as for radar). In sonar the velocity of propagation is the speed of sound (which varies with the nature of the medium in the channel).
Various radar systems and methods are developed to exploit these well-known attributes to measure the range or velocity of targets in different environments. For example, a prior art system 100 that is used to measure the range and velocity of objects is shown in FIG. 2. As is described below in more detail, the radar system 100 is a homodyned frequency shift keyed ("FSK") diplex radar system. As shown in FIG. 2, the system 100 includes a signal generator or oscillator 101, a transmit antenna 102, a transmit coupler 103, a receive antenna 106, a mixer 104, a switch 108, a dual anti-alias filter 105, and a signal processor 107. The signal generator 101 alternately generates two transmit signals: s.sub.1 (t)=Cos((.omega..sub.o +.omega..sub.1)t-.theta.0) and s.sub.2 (t)=Cos((.omega..sub.o -.omega..sub.1)t-.theta..sub.0). The signal generator 101 is thus a diplexed signal generator that alternates between the generation of the s.sub.1 (t) and s.sub.2 (t) signals. The transmit signals s.sub.1 (t) and s.sub.2 (t) are transmitted by the transmit antenna 102 via the transmit coupler 103. The receive antenna 106 receives the reflected signals s.sub.r (t) from target objects where the signals are in the form of s(t-.tau.) (switching between s.sub.1 (t-.tau.) and s.sub.2 (t-.tau.)). Accordingly, s.sub.r (t) is equal to either: EQU Cos((.omega..sub.o +.omega..sub.1)(t-.tau.)-.theta..sub.0) or Cos((.omega..sub.o -.omega..sub.1))(t-.tau.)-.theta..sub.0).
The received signal s.sub.r (t) and the transmit signals s.sub.1 (t) and s.sub.2 (t) are down converted (mixed and low-pass-filtered) by the mixer 104 with the "local oscillator" ("LO") signal Cos((.omega..sub.o +.omega..sub.1)t) and Cos((.omega..sub.o -.omega..sub.1)t). The variable .theta..sub.0 represents the phase delay of the signal between the transmit antenna 102 and the mixer 104 LO signal. The resultant signal is the low pass filter ("LPF") of s.sub.r (t).times.s.sub.1 (t) or s.sub.2 (t), which is either: EQU LPF {Cos((.omega..sub.o +.omega..sub.1)t)Cos((.omega..sub.o.omega..sub. 1)(t-.tau.)-.theta.0)}=Cos((.omega..sub.o +.omega..sub.1).tau.+.theta..sub.0) Eq.1 EQU LPF {Cos((.omega..sub.o -.omega..sub.1)t)Cos((.omega..sub.o -.omega..sub.1)) (t-.tau.)-.theta..sub.0)}=Cos(.omega..sub.o -.omega..sub.1).tau.+.theta..sub.0). Eq.2
The switch 108 is synchronized to the changes in frequency at the diplexed transmit signal generator 101 and thus generates two different outputs at ports 110 and 112 having signals, F1 and F2 nominally equal to Eq. 1 and Eq. 2 after anti-alias filtering by the dual anti-alias filter 105.
In the above equations, ".tau." is the round trip propagation delay to the target. By substituting .tau.=(2/c)(R+Vt) and by letting .omega..sub.d =.omega..sub.o)(2V/c) (note that the Doppler frequency is .function..sub.d =2V.function..sub.0 /c), .theta..sub.0 '=.omega..sub.o (2R/c)+.theta..sub.0, .omega..sub.1 '=.omega..sup.1 (1-(2V/c)).apprxeq..omega..sub.1, then .omega..sub..omega..sub.o.tau.+.theta..sub.0 =.omega..sub.o (2V/c)t+.omega..sub.o (2R/c)+.theta..sub.0 =.omega..sub.d t+.theta..sub.0 ', and .omega..sub.1.tau.+.theta..sub.1 =.omega..sub.1 (2V/c)t+.omega..sub.1 (2R/c)+.theta..sub.1 =.omega..sub.1 (2V/c)t+.theta..sub.1 +2.omega..sub.1 R/c=.theta..sub.1 +=2.omega..sub.1 R/c. Therefore the equations that were written in terms of .tau. can also be written as: EQU F2Cos((.omega..sub.d t+.theta..sub.0 '+2.omega..sub.1 R/c)) and EQU F1=Cos(.omega..sub.d t+.theta..sub.0 '-2.omega..sub.1 R/c)).
Thus, the F1 and F2 signals of the radar system 100 have the same amplitude and frequency but have a different phase. The phase difference between the F1 and F2 signals is .DELTA..phi.=2.omega..sub.1.tau.=2(2.omega..sub.1 R/c)=(4.pi.(2.function..sub.1)R/c). Accordingly for this system 100, the range R is computed by the signal processor 107 as follows: R=(.DELTA..phi.)c/(4.pi.(.DELTA..function.)) where .DELTA..function.=2.function..sub.1 is commonly called the "deviation frequency". Targets of the prior art system (real FSK diplex Doppler radar) appear as signals of the form Cos(.omega..sub.d t+.theta..sub.0 '-2.omega..sub.1 R/c))=Cos(.omega..sub.o (2V/c)t+.theta..sub.0 '-2.omega..sub.1 R/c)).
For outbound targets, i.e., targets with increasing range with time, the Doppler shift .function..sub.d is negative. For inbound targets, i.e., targets with decreasing range with time, the Doppler shift .function..sub.d is positive. The FFT spectrum for real receivers, however, is always symmetrical about its origin. Specifically, the negative frequency portion of the spectrum is equal to the complex conjugate of the positive frequency portion of the spectrum. It is because of this symmetry that target Doppler signals appearing in any Doppler bin may either be inbound targets or outbound targets, thus there exists a velocity direction ambiguity.
Since the two halves of the spectrum in real receivers contain essentially the same information it is customary in real receivers to only process target information in only one half of the spectrum, e.g., in the positive frequency portion of the spectrum. In the prior art system 101 the direction ambiguity is resolved by observing the polarity of the measured delta phase. Since it is known that target ranges must always be positive it can be inferred whether the target information corresponds to an inbound or outbound target. It must be pointed out that resolving this ambiguity does not resolve inbound and outbound targets in the sense of having independent measurements. It is a weakness of the prior art system that the information for two targets with the same Doppler frequency, e.g., one inbound at +.function..sub.d and one outbound at -.function..sub.d, will have their information appearing in the same FFT Doppler bin, resulting in a single corrupted measurement. The resulting measurement cannot be independent for each target since there is only one measurement. If it were possible for the Doppler information for each target to appear in separate FFT Doppler bins then the two targets would actually be resolved in the sense of having independent measurements for each target.
As described above, in homodyned FSK radars, the transmit signal is alternated between a first frequency .function..sub.0 +.function..sub.1 and a second frequency .function..sub.1 -.function..sub.1 signal by the signal generator 101. The signal generator 101 is commonly implemented using a Gunn oscillator. In operation, an external voltage biases the Gunn oscillator or a varactor diode is used to tune the Gunn's frequency. The voltage is varied between two values to generate the s.sub.1 (t) and s.sub.2 (t) transmit signals. Any changes to the deviation frequency creates errors in the range calculations for the system 100. Changes to the deviation frequency may occur due to temperature variations or aging of the oscillator 101.
Radars may be utilized in many different applications. In some applications, it may desirable to be able to determine the range of a target that has zero relative velocity. Such a system may be desirable when used in conjunction with a cruise control system in a vehicle or a side facing radar to detect vehicles in adjacent lanes. Given the equations provided above, it is apparent that the prior radar system 100 is unable to determine the range of a target having zero relative velocity since the phase of the DC Doppler return voltage cannot be measured. In some applications for the radar system 100 this limitation may be undesirable or unacceptable. Another common problem with the performance from the prior art is that the diode mixers that are commonly employed as mixers in radar systems (such as the mixer 104) generate excessive low frequency noise. The range information present in the F1 and F2 signals of the prior art system 100 also occurs at low frequencies for these applications. Consequently these signals may become corrupted or distorted.
In addition to being unable to determine the range of a target having zero relative velocity, the prior art system 100 also has difficulty determining the range of "fading targets". A target appears as a fading target to a radar system when the radar signal reflected by the target has multiple reflections off the target such as from different points along the surface of a target. The numerous reflections of the signal that are reflected by the target generate constructive and destructive interference. In particular, the reception of multiple signals reflected from a single target can distort the phase of the received signal. In the prior art system 100 shown in FIG. 2, such a distortion of the phase also distorts or limits the accuracy of range determinations.
Finally, the prior art system 100 of FIG. 2 may not be able to resolve range ambiguities. Target range is calculated by a phase measurement. All phase measurements are ambiguous in multiples of 360.degree.. Therefore, it is possible for the prior art system 100 to detect a target and calculate its range with a large range ambiguity. Consequently, a need exists for a radar system that can accurately determine the range of targets with little ambiguity.